I just recently did the 18v battery upgrade and after five minutes of using it the control board for the Bluetooth remote started smoking. Should I go ahead and replace it and what else do I need to do

You REALLY should just order one of CJB's 18v upgrade boards, they have a 12 tap to run accessories, but you'll probably have to trace the wiring down to see if you'll have to wire-in relays to handle the 18v to the motors! I've got 4 so far, and the turbo timer's also help to smooth the shock to the gearboxs.

Two reasons I would use a voltage regulator for this:(A) You don't get uneven draining of the batteries. if you wire 3x 6V batteries in series and hook lights and relays up to the lower two, they will drain faster than the "top" one which is only used for the motor, and you'll be required to do individual (balance) charging every time, instead of just occasionally.(B) You can get odd voltages (eg. you want your lights to run at 13.5V so they're brighter). And it will stay consistent even as the batteries drain down(C) you can change the input-side topology (eg. from 3x6V to a 5S Lithium Ion battery) and it will still work.

The cons: They don't like getting wet (and/or you have to spend more money to get a waterproof one)... And they are money.

I think this is up to the user as to what they need it for, but your point is valid -- for a 3x6v setup there are other options.